FIG. 1 is a schematic diagram of the frame structure of a downlink frame of a GPON system in relevant art, and as shown in FIG. 1, in the GPON protocol, one downlink frame includes two portions, PCBd and GTC payload. In this case, the PCBd is composed of portions such as a physical synchronization (PSync) domain, an identifier (Ident) domain, a PLOAMD domain, wherein the PSync domain is 4 bytes, the default value thereof is 0XB6AB31E0, and the Ident domain is 4 bytes; in order to facilitate to achieve synchronization of downlink frames, scrambling code processing is not carried out on the Psync domain; and when the forward error correction (abbreviated as FEC) encoding is carried out, the data structure of the PCBd domain will not be destructed.
FIG. 2 is a schematic diagram of the state conversion of a synchronization state machine of GPON downlink frames in relevant art, wherein ONU is under the searching (Hunt) state at the beginning, and under the Hunt state, the ONU hunts for the Psync domain bit by bit and byte by byte. Once a right Psync is found, the ONU enters the pre-synchronization (Pre-Sync) state and sets a counter N=1. Then, the ONU hunts for the next Psync every 125 us. Each time a right Psync is found, the value of the counter increases by 1. If a wrong Psync is found, then the ONU returns to the Hunt state. Under the Pre-Sync state, if the value of the counter is M1, then the ONU enters the synchronization (Sync) state. After having entered the Sync state, the ONU states that the downlink frame structure has been found and starts to process the PCBd information and Payload information; if wrong Psync are detected for consecutive M2 times, then the ONU states that the downlink frame delimitation is lost and returns back to the Hunt state.
FIG. 3 is a structural schematic diagram of the Ident domain of the GPON downlink frame in relevant art, and as shown in FIG. 3, lower 30 bits in 4 bytes of the Ident domain are a multi-frame counter, the counting value of the Ident domain of each frame is bigger than a previous frame by 1, and when the counter reaches the maximum, the next frame is set as zero. The multi-frame counter can be used for encrypting the user data and can also be used for providing a synchronization reference signal with relatively low rate to the system. In order to tolerate errors for the Ident domain, the ONU has to implement a local multi-frame counter and a multi-frame synchronization state machine. The multi-frame synchronization state machine is the same as the synchronization state machine described in the Psync domain. Under the Hunt state, the ONU loads the received multi-frame counter in the Ident domain into the local counter. Under the Pre-sync and Sync states, the ONU compares the local value and the received value of the counter, if they are matched with each other, then it indicates that the downlink frame synchronization is right, and if they are mismatched, then it indicates that the downlink frame is asynchronous or the transmission is wrong.
In the GPON system, the transmission rate is relatively low, however, in the 10 G GPON, the downlink rate is increased from the original 2.5 Gbps to 10 Gbps, and the bit error rate of the frame header during transmission will be greatly increased, and when the number of errors of the Psync domain or Ident domain accumulates to a certain number, the ONU will enter an asynchronous state, which will cause instant breaking of the ONU downlink data, thus affecting the normal operating of the whole GPON system.
As to the problem in relevant art that in the GPON system since the bit error rate of the frame header is relatively high, it causes the optical network terminal to enter the asynchronous state and causes the reliability of system transmission data to be relatively poor, currently still no effective solution is proposed.